Double metal cyanide compounds are well known catalysts for epoxide polymerization. The catalysts are highly active, and give polyether polyols that have low unsaturation compared with similar polyols made using conventional base catalysis. Conventional DMC catalysts are prepared by reacting aqueous solutions of metal salts and metal cyanide salts to form a precipitate of the DMC compound. The catalysts can be used to make a variety of polymer products including polyether, polyester, and polyetherester polyols. Many of the polyols are useful in various polyurethane coatings, elastomers, sealants, foams, and adhesives.
Conventional double metal cyanide catalysts are usually prepared in the presence of a low molecular weight complexing ether such as glyme. The ether complexes with the DMC compound, and favorably impacts the activity of the catalyst for epoxide polymerization. In one conventional preparation, aqueous solutions of zinc chloride (excess) and potassium hexacyanocobaltate are combined. The resulting precipitate of zinc hexacyanocobaltate is combined with aqueous glyme (dimethoxyethane). An active catalyst is obtained that has the formula: EQU Zn.sub.3 [Co(CN).sub.6 ].sub.2 .multidot.xZnCl.sub.2 .multidot.yH.sub.2 O.multidot.zGlyme
One drawback of conventional DMC catalysts is that they become finely dispersed or practically soluble in polyether polyol mixtures during epoxide polymerizations. Removal of the catalyst from the polyol following polymerization is desirable because the catalyst residues promote an undesirable accumulation of volatile materials (such as propionaldehyde) in the polyol during storage. Unfortunately, DMC catalyst residues are often difficult to remove completely from the polyols, and common catalyst removal techniques usually deactivate DMC catalysts.
Double metal cyanide catalysts usually have good activity for epoxide polymerizations. However, because the catalysts are rather expensive, catalysts with improved activity are desirable because reduced catalyst levels could be used.
Finally, DMC catalysts normally require an "induction" period. In contrast to basic catalysts, DMC catalysts ordinarily will not begin polymerizing epoxides immediately following exposure of epoxide and starter polyol to the catalyst. Instead, the catalyst needs to be activated with a small proportion of epoxide before it becomes safe to begin continuously adding the remaining epoxide. Induction periods of an hour or more are typical yet costly in terms of increased cycle time in a polyol production facility. Reduction or elimination of the induction period is desirable.
Improved DMC catalysts are needed. Catalysts having higher activity are desirable for improving productivity and reducing process cost. Double metal cyanide catalysts with shorter induction periods in epoxide polymerizations would permit a safer, more productive process. Preferred catalysts could be isolated from the polyether polyol product following synthesis and could be reused for additional polymerizations.